This exactly. Try for yourself. Personally I find myself more creative when I don't use caffeine - being slightly tired may even help a little. However, being slightly tired does not help with my more normal, non-creative, work.
A former coworker was just the opposite - put a few espressos in him and the creativity almost oozed out.
The idea is that some types of problems are best solved with the focus you can provide at your peak times of day. However, "insight" problems might benefit from the reduced focus you have at non-peak times.
A former coworker was just the opposite - put a few espressos in him and the creativity almost oozed out.